The industry concerned with power lawn mowers has long been aware of the hazards presented by a rapidly rotating mower blade and of the fact that mower blade accidents can result from failure to appreciate that the momentum of a mower blade tends to keep it in rotation for a substantial period of time after it is declutched from the engine that drives it.
Concern about this hazard has stimulated a constant and diligent search for safety measures that will eliminate mower blade accidents. A number of mower blade brakes have been devised, intended to arrest rotation of a mower blade promptly upon its being declutched from the prime mover that drives it.
U.S. Pat. No. 4,213,288 discloses a rotary mower that has a single blade which is driven through a belt transmission. A belt tightener is arranged to have its idler pulley engage the inside of the belt for tensioning around the driving and driven pulleys. The lever arm that carries the belt tensioning idler pulley also carries a friction pad. As the idler pulley is swung away from tensioning engagement with the belt, the friction pad comes into braking engagement with a portion of the driven pulley circumference that is between the belt stretches which extend to it. This arrangement, although simple, has the disadvantage that the belt tensioner, by reason of its engaging the inside of the belt, decreases the wrap of the belt around the driving and driven pulleys when it tensions the belt. The apparatus is also not well suited to a mower having more than one blade.
U.S. Pat. Nos. 4,186,545, 4,307,558 and 4,409,779 all disclose arrangements which are somewhat more complicated but wherein braking is likewise effected by friction against a driven pulley. These, too, are basically unsuitable for a mower having more than one blade.
U.S. Pat. Nos. 4,395,865 and 4,429,515 disclose a multiple-blade rotary mower having each of its laterally outermost blades mounted on a so-called wing which can be swung up to a position at right angles to the main central portion of the mower. Raising a wing slackens the belt that drives its blade, and belt slackening, in turn, allows a brake bar to be engaged under bias against rim portions of the driven pulley for the blade on that wing. Here, again, the braking system is effective for only one blade, and it is noteworthy that the patent discloses no provision for braking of the blades of the central portion of the mower when those blades are declutched from the prime mover.
U.S. Pat. No. 4,231,215 discloses a rotary mower having three blades. A drive belt trained around a driving pulley and a driven pulley for each blade is tensioned for power transmission by means of a generally conventional belt tensioner. When belt tension is relaxed to declutch the blades from the prime mover, a braking member is moved into engagement with the belt at its zone of engagement with the driven pulley for one of the blades, stopping coasting of the belt and, through the belt, braking the driven pulleys. Because the braking member presents a straight surface to the portion of the belt that it engages, whereas that portion of the belt is curved around a pulley, only relatively small surface areas of the belt and of the friction member are in contact with one another during braking, and therefore the belt is subjected to substantial wear at each braking operation if the brake apparatus is to be effective for rapidly decelerating all three blades
A variant of the arrangement of U.S. Pat. No. 4,231,215 is disclosed in U.S. Pat. No. 4,068,452, which relates to a mower having only two blades. In this case the braking member comprises an elongated rod that is moved laterally for braking to bring its straight end portions into engagement with the belt at its zones of engagement with the driven pulleys for both blades. Although providing for substantially more surface area engagement than the apparatus in which the belt is contacted at only one pulley, the arrangement of U.S. Pat. No. 4,068,452 is suitable only for a mower having only two blades.
U.S. Pat. No. 4,551,967 discloses a machine wherein a rotary blade is driven through a belt transmission and wherein the blade, upon its being declutched from the prime mover, is braked by means of a stationary braking element having a V-groove into which the belt is forced laterally by a braking roller. The braking roller is mounted on the end of a lever arm that is biased to swing that roller towards the braking element and into engagement with the belt. The carrier for the belt tensioning idler pulley has a cam connection with the braking roller arm whereby that arm is swung away from the belt when the idler pulley is shifted to its position for tensioning of the belt, and by such shifting the tensioning idler displaces the belt laterally out of the V-groove in the braking element. The tensioner can perform this function because its idler pulley engages the inside of the belt and thus decreases the wrap of the belt around the driving pulley as the belt is tensioned. Furthermore, the tensioner idler pulley engages a portion of the belt that is directly "downstream" from the brake element, and therefore the belt remains in tensioned engagement with that idler during braking. To accommodate these conditions the tensioner is biased to a defined belt relaxing position, and it is drawn to its tensioning position by the operator, who is thus compelled to judge the amount of tension that should be applied to the belt. If the operator exerts too much strength in drawing the tensioner to its tensioning position, the belt may be stretched excessively, whereas insufficient tensioning of the belt permits slippage at the driving pulley. In principle, very satisfactory blade braking can be effected by forcing the belt into a V-groove in a stationary braking member, but the particular braking arrangement disclosed in this patent--which is intended for braking a single edger blade that rotates on a horizontal axis--is not well suited for a multi-blade mower transmission and is in any case relatively complicated and expensive.